1. Field of the Invention
The invention pertains to the field of dry metallized film capacitors and in particular dry metallized polypropylene capacitors for use, for example, in high intensity discharge ballasts, said capacitors having a protective system to prevent the capacitor from rupturing or catching fire under short-circuit conditions. The protective system uses both thermal and electrical fault interruptors.
2. Description of the Prior Art
There is little known prior art either in the printed literature or in the patent literature in the field of dry metallized film capacitors having internal fault interruptors. What is known about the prior art is that internal fault interruptors have generally been either of thermal or pressure sensitive types. A capacitor protected solely by a thermal fault interruptor will be protected only in the event of a low current leakage, in which, with time the internal heat will rise sufficiently to trigger the fault interruptor. In the situation in which the thermal interruptor is used with an AC capacitor in a plastic case, an unacceptable leakage current situation may develop which, prior to the activation of the fault interruptor, can cause the plastic of the case to melt or rupture. An unacceptable leakage current occurs in particular under conditions in which a hot spot is present and the plastic of the case becomes contaminated with metal from the metallized surface of the capacitor film roll. In the event of a direct electrical short, not cleared by the metallized film, very large currents may flow, causing rapid gas expansion or localized heating away from a thermal interruptor, unless interrupted by an electrical fuse before excessive internal or external damage can occur.
The prior patent art on metallized film capacitors with fault interruptors includes British patent No. 1,569,186 which discloses a film capacitor in which the capacitor roll deforms when heated, breaking a contact. The problem with this device is that by the time the internal temperature is high enough to deform the film roll, some other interruption may have occurred, such as a rupturing of the case or a melt down of the plastic case.
U.S. Pat. No. 3,496,432 also pertains to a dry film capacitor relying on temperature to break a contact.
U.S. Pat. No. 3,909,683 teaches a pressure sensitive fault interruptor in a dry film capacitor. It provides a yielding cavity with a fusible portion of one lead wire passing therethrough. In the event of overpressure caused by a fault, at least one contact bridge is separated. Unfortunately, gases accumulate in the cavity and can be ignited by a spark from the breaking of the contact. The device tends to be too unreliable for commercial use.
Until very recently, as a result of changes in UL requirements, no dry film capacitors with fault interruptors have been commercially available, indicating that the few devices disclosed in the prior patent literature have little or no commercial value, despite proposed UL requirements which have been published in capacitor-UL-810.
The literature on wet film capacitors with fault interruptors is extensive; typical recent U.S. patents are U.S. Pat. Nos. 4,398,782 and 4,454,561.
The prior art either in the literature or in commercially available products does not disclose an internal fault interruptor for dry metallized film capacitors which can operate over a wide range of current wherein the interruptor includes both thermal and current sensitive protective devices.